1. Field of the Invention
The present invention relates to photoresist material suitable for lithography used in the fabrication of a semiconductor where far ultraviolet radiation (FUV) having a wavelength equal to or smaller than 220 nm is to be used as an exposure light, and also relates to monomer and polymer of which the photoresist is to be composed.
2. Description of the Related Art
In a field of manufacturing various electronic devices such as a semiconductor device, an electric device is now required to be more highly densified and integrated. Thus, an improved lithography technique for forming a more minute pattern on a substrate is required for satisfying such a requirement.
As one of methods for forming a more minute pattern is known a method in which an exposure light having a shorter wavelength is used for forming a resist pattern. For instance, in a mass production of a 0.25 .mu.m-rule 256M bits dynamic random access memory (DRAM), it is now being studied that as an exposure light source the i-line having a wavelength of 365 .mu.m is replaced with KrF excimer laser having a shorter wavelength than that of the i-line, more specifically, having a wavelength of 248 nm. However, the fabrication of 0.2 .mu.m-rule 1 G bits DRAM which requires a technique enabling more fine processing needs a light source having a shorter wavelength than that of KrF excimer laser to be developed. For that purpose, an ArF excimer laser having a wavelength of 193 nm now attracts attention as a light source to be used in photolithography.
In addition, a resist to be used for minute processing is required to have a high sensitivity as well as a high resolution in response to a design rule getting smaller and smaller. This is because it is necessary to improve cost performance since an excimer laser to be used as a light source is expensive. As one of methods of enhancing a sensitivity of resist is known a chemically amplified resist which utilizes a photo add generator as a sensitizer. For typical instance, Japanese Patent Application No. 2-27660 has suggested a resist comprising a combination of triphenylsulfonium.hexafluoroarsenate and poly (p-tert-butoxycarbonyloxy-.alpha.-methylstyrene). There are many reports about a chemically amplified resist for use with a KrF excimer laser, for instance, one of which is American Chemical Society Symposium Series, 1984, Vol. 242, pages 11-23, reported by Hiroshi Ito and C. Grantwilson. A chemically amplified resist is characterized by the steps of generating proton acid by exposing a sensitizer or a photo acid generator to a light, transferring the thus generated proton acid through a resist solid phase by means of post-exposure heat treatment to thereby amplify chemical reaction of the resist resin up to hundreds of to thousands of times due to the proton acid in a way like catalytic action. Thus, a quite higher sensitivity can be attained relative to prior resist which has an optical reaction efficiency, which is defined as a reaction per a photon, of smaller than one (1).
Presently, most of newly developed resist is a chemically amplified resist, and hence a chemical amplification system has to be adopted in development of a high sensitive material in response to a shorter wavelength of an exposure light source.
In a lithography technique in which there is to be used an excimer laser such as ArF excimer laser, having a wavelength shorter than 220 nm as an exposure light, a resist for forming a minute pattern is required to have new characteristics which prior resist did not have.
The above mentioned new characteristics with respect to a sensitizer or a photo acid generator are (a) high transparency to exposure lights having a wavelength equal to or smaller than 220 nm, and (b) high reaction efficiency or high photo acid generation ability to the same exposure lights.
In order to satisfy those requirements, the inventors had already developed novel photo acid generators and suggested in Japanese Unexamined Patent Publications Nos. 7-25846 and 7-28237 and Japanese Patent Application No. 6-162244. It should be noted that these publications and application are listed herein merely for better understanding of the present invention, and that the inventors do not admit these publications and application as prior art.
The above mentioned new characteristics with respect to constituents of a resin are (a) high transparency to exposure lights having a wavelength equal to or smaller than 220 nm, and (b) etching-resistance.
In a conventional lithography in which there is used g-line having a wavelength of 436 nm, i-line having a wavelength of 365 nm or KrF excimer laser having a wavelength of 248 nm, a resin constituent of photoresist material is selected from resins which include aromatic rings in a unit structure, such as novolac resin and poly (p-vinylphenol). The dry etching resistance of such aromatic rings gives a dry etching resistance to the above mentioned resin.
However, the aromatic rings quite intensively absorbs lights having a wavelength equal to or shorter than 220 nm, and hence the above mentioned conventional resins cannot be applied to a photolithography which uses an exposure light having a wavelength equal to or shorter than 220 nm. Specifically, most of exposure lights is absorbed at a surface of a resist, and accordingly cannot reach a substrate with the result that a minute resist pattern cannot be formed. Such a fact has been reported, for instance, in "ArF excimer laser lithography (3)--Evaluation of Resist--", the 36th Applied Physics Institution Conference Manuscripts, 1989, 1p-K-4 by Sasago et al. Thus, there is an eager demand of resin materials which do not include aromatic rings, but have a dry etching resistance.
For instance, as a polymer having a transparency to ArF excimer laser having a wavelength of 193 nm and further having a dry-etching resistance, there has been reported a copolymer having adamantylmethacrylate units which are alicyclic polymer in Journal of Photopolymer Science and Technology, 1994, Vol. 7, No. 1, pp 31-40, by Takahashi et al., and Japanese Unexamined Patent Publication No. 5-265212. For another instance, a copolymer having isobornylmethacrylate units has been suggested in Journal of Photopolymer Science and Technology, 1995, Vol. 8, No. 4, pp 623-636, by R. D. Allen et al.
However, the former resin does not have a residue in a residue including therein adamantane and having a dry etching characteristic or in a residue including therein isobornyl, which resin is capable of indicating a difference in solubility between pre-exposure anal post-exposure. In addition, these alicyclic alkyl groups do not include a residue such as a carboxyl group which has solubility to alkaline aqueous solution and a characteristic of adhesion to a substrate. Thus, a polymer composed only of a monomer including alicyclic alkyl groups exhibits high hydrophobic property, and hence also exhibit a weak adhesion to a silicon substrate. Hence, it was quite difficult to form uniform coverage with high reproducibility. In addition, since the above mentioned polymer does not have a residue which can exhibit a difference in solubility between pre-exposure and post-exposure, it is not possible to form a resist pattern by exposure to light. Accordingly, the former resin can be used as a resin constituent of a resist only when they are copolymerized with a comonomer capable of indicating a difference in solubility, such as 3-oxocyclohexylmethacrylate and t-butylmethacrylate, or with a comonomer having an adhesion property to a substrate such as methacrylic acid. Furthermore, since a content ratio of comonomer is required to be approximately 50 mol % and a comonomer unit such as tert-butylmethacrylate and methacrylic acid has quite low dry etching resistance, the dry etching resistance effect caused by adamantane or isobornane skeleton is remarkably deteriorated with the result that the above mentioned resins are not practical as dry etching resistant resin.
Thus, there is demanded a novel material of which photoresist is to be composed, which material has a high transparency to a light having a wavelength equal to or shorter than 220 nm, a high dry etching resistance, a functional group capable of indicating a difference in solubility between pre-exposure and post-exposure, ability of being developed with alkaline aqueous solution subsequently to exposure, and an enhanced adhesion to a substrate.
Other resists than the above mentioned ones and various lithography techniques have been reported as follows.
In "Challenges in Excimer Laser Lithography for 256M DRAM and beyond", International Electron Devices Meeting (IEDM) Technology Digest 1992, pp 45-48, by M. Endo et al., a new chemically amplified positive resist and off-axis illumination technique of KrF excimer laser lithography has been suggested. According to this article, the suggested resist and lithography were successfully applied to the fabrication of 0.25 .mu.m-rule 256M DRAM.
In "Single-layer chemically amplified photoresists for 193-nm lithography", J. Vac. Sci. Technol. B 11(6), November/December 1993, pp 2783-2788, by G. M. Wallraft, there has been suggested a 193 nm single layer resist system based on methacrylate terpolymers. A terpolymer system based on tert-butylmethacrylate, methyl methacrylate and methacrylic acid developed for laser direct imaging applications has been adopted for high resolution imaging by modifying the terpolymer composition and by optimizing the photoacid generator loading.
In "Single Layer Resists with Enhanced Etch Resistance for 193 nm Lithography", Journal of Photopolymer Science and Technology, Vol. 7, No. 3, 1994, pp 507-516, by Robert D. Allen et al., there has been suggested a positive chemically amplified (CA) resist for 193 nm lithography. This paper discusses structure/property relationships of methacrylate polymers with increased etch resistance over the first generation resist.
In "Highly Transparent Chemically Amplified ArF Excimer Laser Resists by Absorption Band Shift for 193 nm Wavelength", Jpn. J. Appl. Phys. Vol. 33 (1994), pp 7028-7032, Part 1, No. 12B, December 1994, by Takuya NAITO et al., naphthalene-containing chemically amplified resists for ArF excimer laser have been proposed, based on the concept of the absorption band shift by conjugation extension. According to the article, the sensitivity of the proposed resist is 150 mJ/cm.sup.2, which is 20 times greater than that of poly (methylmethacrylate) (PMMA).
In "Alicyclic Polymer for ArF and KrF Excimer Resist Based on Chemical Amplification", Journal of Photopolymer Science and Technology, Vol. 5, No. 3 (1992), pp 439-446, by Satoshi Takechi et al., it has been reported that the polymers having an alicyclic hydrocarbon show dry-etch resistance comparable to Novolac resists. Based on this discovery, a new chemical amplification resist has been proposed for ArF and KrF excimer lithography. The resist comprises the copolymer of adamantylmethacrylate and tert-butylmethacrylate with triphenylsulfonium hexafluoroantimonate as a photo acid generator. According to the report, the use of the proposed resist enables 0.40 micron lines and spaces using KrF excimer stepper.